1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator (TCXO), and more particularly, to a temperature-compensated crystal oscillator which maintains a satisfactory phase noise characteristic and facilitates adjustments to its oscillation frequency.
2. Description of the Related Arts
A temperature-compensated crystal oscillator compensates variations in frequency due to a frequency-temperature characteristic of a quartz crystal unit to stably maintain the oscillation frequency notwithstanding a change in ambient temperature. Such a temperature-compensated crystal oscillator is widely used as a frequency source particularly in a portable telephone and the like which is used in a mobile environment. In recent years, the temperature-compensated crystal oscillator is required to excel in the phase noise characteristic, needless to say that it must be small in size and low in cost.
FIG. 1 illustrates an exemplary configuration of a conventional temperature-compensated crystal oscillator of a voltage-controlled type. The illustrated temperature-compensated crystal oscillator comprises voltage-controlled crystal oscillator (VCXO) 1; compensating voltage generator (COMP) 2 for generating a compensating voltage which is applied to the voltage-controlled crystal oscillator 1 as a control voltage; low pass filter (LPF) 3; and switching element 4. Voltage-controlled crystal oscillator 1 generally comprises crystal unit 5; two voltage variable capacitance elements connected to both ends of crystal unit 5, respectively, to form a resonator circuit together with crystal unit 5; and inverter 7 for oscillation which amplifies a current of the resonator circuit and feeds back the amplified current. Variable capacitance diodes 6 are used herein for the voltage variable capacitance elements, and each has a grounded anode and a cathode connected to crystal unit 5. Feedback resistor 8 is inserted to connect an input terminal to an output terminal of inverter 7. The input terminal and output terminal of inverter 7 are connected to the resonator circuit, respectively, through DC blocking capacitors 9. Oscillation output Vout is generated from the output terminal of inverter 7.
Crystal unit 5 is made, for example, of an AT-cut quartz crystal blank, and its frequency-temperature characteristic is represented by a cubic function curve as indicated by curve A in FIG. 2. In FIG. 2, the vertical axis represents a frequency deviation xcex94f/f, where f is the frequency at 25xc2x0 C. Due to the use of such crystal unit 5 having the frequency-temperature characteristic as illustrated, the crystal oscillator also exhibits a similar frequency-temperature characteristic in its oscillation frequency.
Compensating voltage generator 2 comprises a temperature sensor connected to power supply Vcc for detecting, for example, an ambient temperature; and a cubic function generator for generating a voltage which changes in accordance with a cubic function in response to a detected temperature. Compensating voltage generator 2 generates compensating voltage Vc in response to the ambient temperature. Compensating voltage Vc is applied to the cathode of each variable capacitance diode 6 through high frequency blocking resistor 10. Each of variable capacitance diode 6 changes the capacitance between the anode and cathode terminals in response to compensating voltage Vc applied thereto, resulting in a change in an equivalent series capacitance (load capacitance) viewed from crystal unit 5 to cause a change in the oscillation frequency. Compensating voltage Vc is set herein to change in response to the temperature in accordance with a cubic function curve, as indicated by curve B in FIG. 2, to compensate the crystal oscillator for the frequency-temperature characteristic so that the characteristic curve becomes flat.
Low pass filter 3, which comprises a CR time constant circuit composed of capacitor (C) 11 and resistor (R) 12, is inserted between voltage-controlled crystal oscillator 1 and compensating voltage generator 2. Compensating voltage Vc from compensating voltage generator 2 is applied to variable capacitance diodes 6 through low pass filter 3, thereby removing low frequency noise components possibly included in compensating voltage Vc.
Low pass filter 3 comprising a CR time constant circuit, provided as mentioned above, causes a delay in applying compensating voltage Vc to variable capacitance diodes 6 upon start of the temperature-compensated crystal oscillator, thereby exacerbating the starting characteristic of the oscillator. To solve this problem, the temperature-compensated crystal oscillator has switching element 4 connected in parallel with low pass filter 3 such that low pass filter 3 is short-circuited upon starting. Specifically, switching element 4 is connected in parallel with resistor 12 of the time constant circuit, and short-circuits resistor 12 only in the event of starting the temperature-compensated crystal oscillator, and is turned off after the starting. This avoids the delay in the operation upon starting due to the CR time constant circuit, resulting in a satisfactory starting characteristic. Switching element 4 is controlled by starting control circuit (START CNTL) 13 connected to power supply Vcc.
Starting control circuit 13 comprises PNP transistor 18 connected between power supply Vcc and switching element 4; resistor 19 connected between the base of PNP transistor 18 and a ground point; and capacitor 20 connected between the base and power supply Vcc, for example, as illustrated in FIG. 3. With this circuit configuration, PNP transistor 18 conducts to turn on switching element 4 when the crystal oscillator is powered on. Subsequently, the base voltage increases to the same potential as power supply Vcc by a time constant determined by capacitor 20 and resistor 19 to turn off PNP transistor 18, thereby turning off switching element 4.
The respective circuits, which form the temperature-compensated crystal oscillator, are generally integrated in a single IC (integrated circuit) chip except for crystal unit 5.
Then, the temperature-compensated crystal oscillator is assembled, for example, as illustrated in FIG. 4, by securing IC chip 15 on the bottom of a recess formed in container body 14 made of laminated ceramic, for example, by face down bonding, and securing one end of quartz crystal blank 5A, which constitutes crystal unit 5, on a step formed on the recess with a conductive adhesive. Excitation electrodes 17 are formed on both main surfaces of crystal piece 5A. Then, one of excitation electrodes 17 on crystal blank 5A is irradiated with an ion beam, as indicated by arrow P in FIG. 4, to reduce the thickness of excitation electrode 17 in order to adjust the oscillation frequency of the oscillator, such that the oscillation frequency matches a reference frequency. Stated another way, the oscillation frequency is adjusted by decreasing an additional mass to crystal unit 5. The reference frequency used herein refers to a so-called nominal frequency at which oscillation should be carried out, for example, at a room temperature. After the adjustment is completed for the oscillation frequency, a lid member is placed to cover the recess of container body 14 to encapsulate crystal blank 5A and IC chip 15 within the recess, thereby completing the temperature-compensated crystal oscillator. A circuit pattern is formed on the surface of the recess of container body 14 for electrically connecting IC chip 15 to crystal blank 5A, and electrode portions are formed on the outer surface of container body 14 for connecting the temperature-compensated crystal oscillator to an external circuit.
However, the temperature-compensated crystal oscillator in the foregoing configuration suffers from the inability to accurately match the oscillation frequency with the reference frequency during the adjustment. As one excitation electrode 17 of crystal blank 5A is irradiated with an ion beam, which is a charged particle beam, an electron flow (i.e., electric current) is generated. This current introduces into low pass filter 3 through high frequency blocking resistor 10, and causes a voltage drop particularly by resistor 12 in the time constant circuit of low pass filter 3. As such, compensating voltage Vc which corresponds to the reference frequency and is supplied from compensating voltage generator 2 varies when it is applied to variable capacitance diodes 6. Consequently, the adjusted oscillation frequency will shift from the reference frequency even if the ambient temperature is normal.
While the shift in frequency may be previously taken into consideration for the adjustment, the shift largely varies in amount, failing to provide a practical radical solution. It should be noted that high frequency blocking resistor 10 has approximately 100 kxcexa9, which is too small as compared with resistor 12 of the time constant circuit which has approximately 2 Mxcexa9, so that a voltage drop caused by resistor 10 does not particularly causes a problem. While capacitor 11 having a larger capacitance may be employed for the time constant circuit of low pass filter 3 to allow for using resistor 12 having a smaller resistance, such capacitor 11 would occupy a prohibitively large area, thus experiencing difficulties in the integration into an IC chip.
It is an object of the present invention to provide a temperature-compensated crystal oscillator which can be securely adjusted for the oscillation frequency during the manufacturing.
The object of the present invention is achieved by a temperature-compensated crystal oscillator which comprises a voltage-controlled crystal oscillator having a crystal unit, a compensating voltage generator for generating a compensating voltage applied to the voltage-controlled crystal oscillator, a low pass filter inserted between the compensating voltage generator and the voltage-controlled crystal oscillator, a switching element connected in parallel with the low pass filter for short-circuiting across the low pass filter, and control means for controlling the switching element into a conducting state when an excitation electrode of the crystal oscillator is irradiated with an ion beam for adjusting an oscillation frequency.
Specifically, the temperature-compensated crystal oscillator of the present invention has the switching element in parallel with the low pass filter inserted between the compensating voltage generator and voltage-controlled crystal oscillator. When the excitation electrode of the crystal unit is irradiated with an ion beam for adjusting the oscillation frequency, this switching element is made conductive to prevent a change in the compensating voltage supplied from the compensating voltage generator to the voltage-controlled crystal oscillator, because no voltage drop is caused by the low pass filter even if an electron flow is generated during a frequency adjustment. As a result, the adjusted oscillation frequency is prevented from shifting from a reference frequency.